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I know you were tempted to start coloring but you should have just finished your line work before moving on. I say this because your drawing is heavily reliant on its lines.
From my understanding you're viewing each pokemon as individual parts and focusing on one pokemon at a time. While thats not necessarily a bad thing, if you want that extra 'umpf' and 'pizazz' to distinguish this from other drawings you need to view it as a whole.
Get into the mindset that the line work is one piece and when that is absolutely perfect, then move onto the color. You've rushed onto flat colors without taking notice of the inconsistent line work. I'm not insulting you, I'm pointing out a glaring flaw, and if you want to improve then you need to pay attention to the details.
In the picture I've cleaned up the lines and placed a comparison of the original next to it. This is the kind of polish that needs to be present thorough the piece.
Each pokemon are wonderfully drawn, though you may need to revise some areas around Jolteon's design. There are many tangents and he begins to lose his form. Big example is where his chin and paw meet.
I don't know how you plan on coloring this but I suggest you taper off areas of the pokemon further away from us in order to create a sense of depth. It could be as simple as brushing over layers of white once it's completed in order to make your Pokemon 'pop' out more. Also I suggest a simple background.
Good job, I'm sure your bf would enjoy it.